1. Field of the Invention
The present invention relates to a method of communicating between two stations operable at high data transfer rates with low system overhead, and the apparatus used in accomplishing the communication of the present invention.
2. Background Art
Various techniques for transferring data between two stations have been proposed. One involves synchronous communication, that is, where the transmitting and receiving stations keep in "lock-step" with each other for each bit of information within the entire packet being transferred. Synchronous communication requires the transmitting station to have all the data for the packet ready before transmission begins, and places a stringent real-time constraint for the continuous transmission of the entire packet. Data link control is employed to manage the communication between two or more stations, sending additional information for such things as station selection and indicating the beginning of a packet, or the end of a packet. Some examples of synchronous communication are binary synchronous communication (BSC), synchronous data link control (SDLC), and high-level data link control (HDLC).
Another technique for transmitting data involves the use of asynchronous, or start-stop communication. In asynchronous communication, synchronization is only maintained for the duration of transfer of each character or byte within the packet being transferred. Additional bits, start-stop bits, must be sent with each character to establish synchronization. Varying time intervals may pass between the transfer of each character in the packet. By its very nature, asynchronous communication is usually considered low speed and inefficient and therefore not conducive to high rates of data transfer Asynchronous communication has the advantage of requiring a low amount of overhead to implement.
Accordingly, prior art communication systems have significant disadvantages and limitations.
In the transfer of image data from a central processor or computer to a printer or other output device, it is desirable to have the high rates of data transfer associated with the synchronous transfer techniques while maintaining continuity of communication and necessary status. It is also desirable to have the low overhead and the convenience of varying transfer intervals of the asynchronous techniques.
Data communication techniques and practices for prior art systems are detailed in a publication "Data Communications Concepts", published by IBM at various times between 1977 and 1987, one version of which is identified as publication GC21-5169-6. This publication is expressly incorporated herein by reference as teaching some of the fundamentals and vocabulary of data communication, and familiarity with this publication is assumed in understanding the teachings of the present invention.